bcache: not use hard coded memset size in bch_cache_accounting_clear()

In stats.c:bch_cache_accounting_clear(), a hard coded number '7' is
used in memset(). It is because in struct cache_stats, there are 7
atomic_t type members. This is not good when new members added into
struct stats, the hard coded number will only clear part of memory.

This patch replaces 'sizeof(unsigned long) * 7' by more generic
'sizeof(struct cache_stats))', to avoid potential error if new
member added into struct cache_stats.

Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/stats.c

@@ -111,7 +111,7 @@ void bch_cache_accounting_clear(struct cache_accounting *acc)
 {
 	memset(&acc->total.cache_hits,
 	       0,
-	       sizeof(unsigned long) * 7);
+	       sizeof(struct cache_stats));
 }
 
 void bch_cache_accounting_destroy(struct cache_accounting *acc)